1. Field of the Invention
The present invention relates generally to methods and apparatus for displaying ECG waveforms and more particularly to such methods and apparatus in which heartbeat waveforms contained in stored ECG data are displayed superimposed over one another.
2. Description of the Related Art
In a Holter ECG monitoring system, a patient is fitted with a monitor which detects and stores continuous ECG waveforms. Sometimes several electrodes are connected to the patient so that two or more channels are recorded. It is not unusual for such a monitor to be worn for many hours thereby generating thousands of periodic heartbeat waveforms which are stored for later review to assist in diagnosis and treatment of the patient.
Sometimes such waveforms are displayed by superimposing a predetermined number of heartbeats on a screen in rapid succession. Each time a new heartbeat appears on the screen, the longest-displayed heartbeat is removed from the screen. Such a display allows a clinician to see beat-to-beat changes in the patient's ECG complex which may be significant. Holter monitors can produce as many as 100,000 consecutive beats which must be analyzed to identify both short-term and long-term changes. When an abnormal complex is displayed, the user is interested in the shape and quantity of abnormal complexes, especially when seen in close proximity to each other. Multiple sequential abnormal complexes are of special importance; the superimposition display is provided to detect such sequences.
Early forms of the superimposition display scanned each beat once on a high persistence phosphor in a cathode ray tube (CRT). The CRT displayed, albeit on a gradually dimming basis, several of the heartbeats at once. Thus, a user was able to observe an integer number of heartbeats where the number of heartbeats observed was determined by the phosphor characteristics of the CRT. More recent systems have been developed which use digital memory to store and display the beats of interest. Some such systems display one beat at a time and rely upon an observer's eye to integrate several successive beats during the search for abnormal complexes and runs thereof.
Some digital systems now provide for a fixed number of beats to be overlaid and displayed simultaneously. One such system displays 12 beats. When a new beat is added to the display, the oldest beat is removed. Sequential new beats are periodically added (and old beats removed) one at a time, to facilitate rapid review of the data. This prior art approach is disadvantageous in that a user cannot easily differentiate between short and long continuous runs of ectopy, or other abnormalities, nor can a user select a threshold to optimize searching for a predetermined number of sequential abnormal complexes.
Another method for displaying stored ECG data is to display a continuous static ECG waveform. Typically the waveform is presented from left to right across the screen in a plurality of rows. A user can page forward or backward through the stored data to display different portions of the waveform on the display screen. Such a display optimizes the ability of a clinician to detect patterns of abnormal heartbeats, such as bigeminy, as well as rate information which is derived from the spacing of each heartbeat waveform.
Thus, the superimposition display provides excellent beat-by-beat detail for the displayed beats but larger patterns of beats, as well as rate information, are lost. In one prior art superimposition display system, some rate information is provided in a superimposition display screen. As each new beat is superimposed on the screen, a horizontal line is drawn on the screen to one side of the waveform display. Each horizontal line is proportional in length to the interval between R-waves of the currently displayed heartbeat waveform and the immediately preceding one. A rough plot of rate information is thus also available to the clinician in association with the superimposition display. The remainder of the waveform data, including any heartbeat waveform patterns which develop, is not presented by this prior art system.